The present invention relates to the manufacture of discrete or integrated semiconductor components and particularly to the fitting thereof within a plastic box.
The invention relates to (assemblies) in a plastic box privided with a planar metal plate forming a heat radiator and provided with connecting pins projecting from the box, the assembly incorporating, within said box, the stacking on the plate forming the radiator of a ceramic plate, a metallic counter-electrode, a semiconductor chip having a rear face in electrical contact with the counter-electrode and a front face having at least one metallized zone, one of the connecting pins electrically being connected to the counter-electrode and at least one other pin being electrically connected to the metallized zone of the front face of the pellet.
This definition of a type of fitting is given in general terms and does not exclude numerous variants particularly the interposing of other metal sheets or plates between the aforementioned elements, e.g. a supplementary counter-electrode between a metallized zone of the front face of the semiconductor pellet and the connecting pin to be electrically connected to said zone.
A fitting of this type is generally known under the name "TO 220 case", whose general appearance is shown in FIG. 1 and which is conventionally used for the fitting of thyristors, transistors or triacs having a medium power level. On this figure, it is possible to see that metal plate 10 forming the radiator with a circular opening 12 for fixing by screws to a larger radiator, the plastic box 14 covering the fitting elements, but leaving free a rear face and part of the front face of the plate forming the radiator and finally three connecting pins 16, 18, 20 projecting from the box essentially in a plane parallel to the plane of plate 10.
This type of fitting is presently carried out by the applicant company in the following way (cf. FIG. 2, which shows the elements prior to assembly). The starting object is a planar radiator plate 10 having locations for several components. This plate will subsequently be cut into individual components. The following elements are successively stacked on the plate 10 for each component: a welding sheet (preform) 22, a ceramic plate 24 which is nickel-coated in its centre 26 on both faces, another welding preform 28, a lead frame 30, which is a metal tape cut so as to form both a counter-electrode 32 and connecting pins 34, 36 and 38, pin 38 being used as the connecting pin connected to counter-electrode 32. The connecting pins are joined to one another and are also joined to pins of other components simultaneously fitted to the same radiator plate in the lead frame 30. This lead frame 30 will subsequently be cut in order to individualise the different pins. The semiconductor chip 42 to be encapsulated is then placed on the stack, by placing its lead solder-coated metallized rear face in contact with counter-electrode 32. The front face of the chip has two metallized zones 44, 46 visible in FIG. 2, e.g. a cathode contact zone and a gate contact zone for a thyristor, said zones also being coated with lead solder. Finally, two conductive bridges 48, 50 are fitted, so that each of then is in contact both with a metallized zone and a respective connecting pin 34, 36.
During this fitting and up to the welding of the thus stacked elements, the welding flux between the different parts acts as a glue in order to maintain said parts in place. The assembly is then placed in a welding furnace (at approximately 320.degree. C.) after which the welded assembly is placed in a mold and coated with thermosetting resin, whilst leaving bare the rear face of the plate forming the radiator. The connecting pins are then separated from one another by cutting and finally the radiator plate is cut up to form the individual components.
Detailed analysis of this fitting process reveals the existence of several disadvantages relating to the actual fitting process and to the component resulting therefrom.
Firstly, the stacking of most of the elements takes place manually, although the positioning thereof is difficult. Sometimes the connecting grid is mechanically positioned with respect to the radiator plate, but this is not the case with the other elements. In particular, it would be very difficult to envisage an automatic positioning of bridges 48 and 50 between the metallized zones and the connecting pins. Thus, the production efficiency is reduced by positioning defects.
When the elements are stacked, it is the welding flux which acts as a glue to keep the elements in their reciprocal positions to the time when they leave the welding furnace. Thus, it is necessary for said flux to be present, but it suffers from the disadvantage of rapidly dirtying the welding furnace and the means supporting the components during their passage in the furnace. Moreover, the presence of flux in the weld leads to a reduction in the quality of the latter, because the flux forms bubbles in the weld, prejudicial to the mechanical quality and the thermal conduction thereof in operation. It should also be noted that the production cost of conductive bridges is high.
Finally, the counter-electrode 32 forming an integral part of the connecting grid necessarily has a surface which is essentially limited to the surface of the semiconductor chip and this is for a number of reasons essentially linked with the fact that counter-electrode is laterally bounded by the connecting pins 34, 36 which surround it. Thus, these pins 34, 36 must be as close as possible to the corresponding metallized zones of the chip. This leads to a limitation to the thermal dissipation possibilities across the counter-electrode, the ceramic plate and the radiator plate with regard to the heat produced in operation in the pellet.
In order to generally improve the box fitting processes and the quality of the semiconductor components resulting therefrom the present invention proposes a novel encapsulated semiconductor component structure and a novel fitting process making it possible to obtain this structure.